responsible for sending unicast packets。 The mobile node’s parent router node is responsible to forward the packet to the gateway。 Other router nodes are not responsible for forwarding this packet, just clipping the mobile node of RSSI value, then forwarded to the gateway。

Finally the gateway bring all RSSI values of the mobile node to PC monitoring software, the corresponding mobile node’s location is calculated。 In order to reduce the error, monitoring software will collect 10 times of the RSSI value and take average on it, and then select the nearest value of the three fixed nodes。 Finally the trilateral measurement method is used to calculate the location of mobile nodes。论文网

V．SYSTEM IMPLEMENTATION

All software systems embedded in nodes are based on Z-Stack。 Because

Z-Stack is an open-source project, it is very beneficial to the secondary development。 These nodes were tested in a real coal mine locate in Shanxi Province。 We deployed the fixed node every 50 meters in the tunnel, and also set a fixed node in each entrance of the work area。 Because the fixed node have large size digital LED displays, so the display content of the fixed node can be seen far from away the miner。 Each miner carries a mobile node, the temperature and gas concentration is displayed on the LCD screen at

real-time。

The gateway node is placed at the entrance of the mine, through the RS232 cable connected to the monitoring computer in the control room。 In this system all packets collected by the gateway node are transmitted to PC through a serial port, and it can save historical data backup to a SQL database。 The main function of monitoring software is to display and store the data of every node, and calculates related mobile nodes’ location according to RSSI values。 The monitoring software has two main dialog interfaces, one is

used to display a two- dimensional profile of the coal mine, and user can see all the miners' working position。

Another interface is data displaying interface, and environmental data were shown here。 The picture of PC monitoring software is shown in Fig。 

Figure 2。 PC monitoring software。

VI．SYSTEM EVALUATION

Through repeated testing of the system, we made the system an objective assessment。 First is the power consumption assess for node hardware, fixed node’s working voltage is in 9V ~ 24V when the power supplied by cable。 The maximum operating current for fixed node is 93mA; the average operating current is 92。2mA。 When the power cable was disconnected, fixed node powered by lithium-ion battery。 On battery power, the fixed node’s maximum working current is 147mA; average working current is 146。3mA。 Fixed nodes can work 8 hours on battery power at least。摘要：ZigBee 是一种的无线个人区域网络标准，应用在需求低功耗的情况，比如无线传感器与控制网络。 由于节点可能被布置在一个交通不便的地方，因此需要高能量效率,最大限度地提高网络的寿命和减少维护 费用。为了提高寿命，一个简单而直接的解决办法是关闭所有不必要的节点，例如，当节点空间密度大于文献综述

检测需要时，就可以关闭那些不必要的节点。我们设想一个新型 ZigBee 无线交互网络，远程传感器节点可 以选择性地关闭。更确切地说，无线控制是基于射频识别（RFID)技术，从而实现一个 ZigBee/RFID 混合 结构。换言之，我们考虑使用两个逻辑重叠的网络，RFID 和 ZIGBEE 网络。RFID 网络，通过一个电源关闭 算法打开或关闭节点的 ZigBee 网络。这个算法被称为深度睡眠算法，旨在平衡各个 ZigBee 节点的剩余能 量。事实上，RFID 控制器（即阅读器）循环开关剩余能量很低的 ZigBee 节点。通过构建所提出的 RFID 控 制 ZIGBEE 技术，我们专注于观测点的需要最少的空间密度实现。这是出于实际利益考虑，比如在一个分布 式监控应用中，其中一个节点需要使用同一种方式监控尽可能大的区域。在这种情况下，我们在监控区域 引进了一种虚拟空间网格，并且在网络中的每个节点应用深度睡眠算法，要求在同一时间最多只有一个节 点是工作的。我们利用基于 Opnet 网络模拟对所提出的 ZigBee-RFID 混合网络进行分析。